The greatest words any American ever said were spoken by a gaunt, war-haunted man in a tiny Pennsylvania college town 150 years ago Tuesday.

The celebrated orator Edward Everett had spoken first, a gusty address that began with a nod toward "this serene sky, overlooking these broad fields now reposing from the labors of the waning year," and wheezed to a close two hours later with a reference to "the glorious annals of our common country." One imagines a sonorous baritone, the "r's" rolling like carriage wheels.

When it was his turn, Abraham Lincoln stood in that cemetery in Gettysburg, a town whose walls were still pitted with bullet holes from the great battle four months before and whose heart was still scarred with the memory of bodies lying twisted, bloated and mangled in the rain. "Four score and seven years ago," he said in that high, piping twang, "our fathers brought forth on this continent a new nation, conceived in liberty and dedicated to the proposition that all men are created equal. Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure."

In a speech lasting just two minutes, he grappled with the challenge of defining America.

It is noteworthy that the second greatest thing any American ever said echoed the first. Standing at Lincoln's shrine a century later, Martin Luther King noted his "symbolic shadow" and his signing of the Emancipation Proclamation "five score years ago." And when he said, "I have a dream," the first dream King articulated was that "one day this nation will rise up and live out the true meaning of its creed: 'We hold these truths to be self-evident, that all men are created equal.'"

Of course, the need to define America outlived Lincoln. And for that matter, King.

That need assumes fresh urgency and new poignancy in a nation where secession is bruited about like some dirty joke, mosques are burned, mass incarceration is practiced, "hatred" is not too strong a word to describe the political atmosphere, and a black boy walking with candy and ice tea in his pockets is deemed worthy of the death penalty.

Obviously, we have work to do. And yet ...

Last month, after exhausting the usual "where are you folks from" patter, the driver of a tour bus in Gettysburg offered this view of the battle that happened there: "Neither side was wrong," he said. "Both fought for their beliefs." He seemed not to consider that the Nazis did, too.

It was an attempt at moral equivalence, a pretense that both sides are equally valid, and it is not uncommon. When offered a chance to define what America means, some of us rush from judgment.

But Gettysburg and the larger war of which it was a turning point were fought to bring together a broken country and end the abominable practice of slavery. No American patriot can countenance moral equivalence on the first cause, no moral man or woman can accept it on the latter. And Lincoln, who had always instinctively embraced the one, was coming, by the time he spoke, to also embrace the other.

Because defining America was never really the challenge: The founders erected their new country upon a clear, albeit revolutionary promise -- equality and freedom. No, the challenge was whether America would ever choose to honor that promise -- a question that still awaits an answer. The "honored dead" of Gettysburg, said Lincoln, gave "the last full measure of devotion" fighting to answer it in the affirmative. This was an act of faith.

Now, he said, the work of validating that faith was left for the rest of us. A century and a half ago, Abraham Lincoln handed this country a trust. It was heavy. And burdensome. And sacred.